How you start the GUI depends on how you installed it.

Chocolatey Install

Did you install the chocolatey package? Chocolatey will have installed a small stub program to run the GUI. In that case just type

testcentric

NuGet Install

If you added the install directory to the path, just type

testcentric

Otherwise, use the path to which the GUI was installed:

<path to the install directory>\testcentric.exe (*)

(*) Of course, you would use a forward slash on Linux.